The present invention relates to cellular communication systems, and more particularly to reducing maximum transmit power in radiocommunication equipment that utilizes a plurality of correlated carriers.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often referred to as “base stations”. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
The radiofrequency spectrum that is utilized to provide mobile communication services is a limited resource that must be shared in some way among all of the users in a system. Therefore, a number of strategies have been developed to prevent one mobile device's use (both transmitting and receiving) of radio spectrum from interfering with that of another, as well as to prevent one cell's communications from interfering with those of another. Some strategies, such as Frequency Division Multiple Access (FDMA) involve allocating certain frequencies to one user to the exclusion of others. Other strategies, such as Time Division Multiple Access (TDMA) involve allowing multiple users to share one or more frequencies, with each user being granted exclusive use of the frequencies only at certain times that are unique to that user. FDMA and TDMA strategies are not mutually exclusive of one another, and many systems employ both strategies together.
Yet another means for sharing radiofrequency resources is Code Division Multiple Access (CDMA). In CDMA, all users can share a radiofrequency resource at the same time. In order to prevent one user's transmissions from interfering with those of another, each pair of users (transmitter and receiver) is allocated one of a set of predefined orthogonal spreading codes. Each spreading code is a sequence of digital bits having a higher bit rate than that of the user's data to be communicated. The transmitting user's digital data is effectively multiplied by the spreading code (e.g., by means of an “exclusive OR”—“XOR”) to create a higher rate stream of bits that can be considered to represent either a “+1” or a “−1”, and it is this higher rate “spread” data that is transmitted over the shared radiofrequency resource.
To receive the underlying data, a receiver correlates the received signal against the same assigned spreading code. Due to orthogonality between spreading codes, the codes of other users will cause their signals to appear as noise, so that the correlation between the assigned spreading code and those user's signals will effectively be zero. Only the signal that the receiver is trying to receive will have a correlation of either “+1” or “−1”, and in this manner the underlying data is extracted from the received signal.
The above discussion presents CDMA concepts in relatively simplistic terms. In practice, there are many other aspects that are employed, which are well known to those of ordinary skill in the art, such as (and not limited to) the combined use of both so-called “channelization codes” (which separate transmissions from a single source) and so-called “scrambling codes” (which separate terminals or base stations from each other).
A number of communication systems have been standardized to include an air interface that relies on CDMA technology. One of these is Wideband-CDMA (WCDMA), which, in the single carrier case, transmits on a pair of 5 MHz-wide radio channels. To facilitate the reader's understanding of the various concepts discussed herein, terminology associated with the well-known WCDMA system is used herein. However, the various concepts discussed herein are not limited to use only in WCDMA systems, but are instead more generally applicable to any communication system having comparable features.
Power control is another important aspect of mobile communication systems. Too little transmission power can prevent a transmitter from being heard over the transmissions of others. By contrast, too much transmission power can not only drown out the transmissions of others, but can also unnecessarily waste power (which is especially detrimental in a battery powered device) and can also put unnecessarily strict requirements on the linearity of the transmitter's circuitry.
The maximum transmit power in WCDMA is the minimum of the Universal Terrestrial Radio Access Network (UTRAN) signaled “allowed” maximum power and the power class determined “nominal” maximum power. The nominal maximum power is defined by the power class. In dual (or multi-) carrier operation, a transmitter simultaneously transmits on two (or more) separately modulated carriers, each of which occupies a distinct frequency region. The maximum user equipment (UE) transmitter power for dual (or multi-) carrier operation is defined as the total power on both (or all) carriers.
The nominal maximum power may be reduced by a configuration dependent amount that is called “maximum power reduction” (“MPR”). (In some texts, this is also referred to as a “back-off metric”.) Allowing the UE to reduce the maximum power makes it easier for the UE to satisfy transmitter requirements in terms of, for example and without limitation, Adjacent Carrier Leakage Ratio (ACLR) (which is a ratio of the power emitted in an intended channel to the power leaked into a certain nearby channel) and Error Vector Magnitude (EVM). This reduces the transmitter linearity requirements, in particular for the power amplifier. This, in turn, leads to reduced costs in terms of chip area and power consumption.
The MPR in WCDMA systems is computed based on a so-called “Cubic metric”. The Cubic metric is the root-mean-square (RMS) value of the cubed waveform (after power normalization) expressed in decibels (dB), reduced by an offset of 1.52 dB, and then scaled by a configuration dependent scale factor. The scale factor is 1.85 for single carrier configurations using only the lower half of the code tree, 1.56 for other single carrier configurations, and 1.66 for dual carrier configurations. The offset and scale factors were selected so that the Cubic metric approximates the required back-off. The Cubic metric definition by the Third Generation Partnership Project (3GPP) also includes a quantization step. The present discussion, however, will consider this quantization instead to be part of the MPR computation, and the Cubic metric will refer to the non-quantized entity.
The Cubic metric depends mainly on the channel configuration, but to some extent also on the scrambling code and the transmitted symbols. The channel configuration is described by the number of different physical channels and their gain factors (power offsets), type of modulation (Binary Phase Shift Keying (“BPSK”), 4-level Phase-Amplitude Modulation (“4PAM”)), branch under consideration (“In-Phase” (“I”) or “Quadrature Phase” (“Q”)), channelization codes and spreading factors. The channel configuration, for the case of dual carriers, is also described by the power offsets between carriers. The branch and channelization code for each physical channel is specified based on the number of different physical channels. This means that the Cubic metric mainly depends on the number of different physical channels, their gain factors, the power offset between carriers, modulation and spreading factors.
The UE needs to know the allowed MPR for each possible configuration. The Cubic metric can be dynamically computed or (to save processing power) precomputed and stored in a lookup table when only single carriers are involved. In theory, the introduction of dual carriers essentially means that each single carrier configuration may be combined with any configuration on the second carrier, for any power offset between the carriers. This would result in about [8.5 million]2 possible dual carrier configurations. However, the standard disallows most combinations so that, in practice, the number of configurations for dual carrier operation increases from about 8.5 million to about 300 million per power offset between carriers. Consequently, when dual carriers are involved, practical embodiments require that the Cubic metric be approximated (i.e., rather than computed). To take just one of a number of possible examples, approximating the Cubic metric can be based on the Cubic metrics of the corresponding carriers.
As disclosed in U.S. Patent Publication No. US-2010/0239031, which is hereby incorporated herein by reference in its entirety, the Cubic metric for dual carriers can be well approximated based on the per carrier Cubic metric using an affine map. Such a map can be parameterized in several possible ways. Herein, the following map is used:k·CM=a0+a1·k1·CM1+a2·k2·CM2  (1)where the parameters a0, a1, a2 vary with the relative power difference between the carriers and the scaling factors k, k1, k2 are the scaling factors specified by 3GPP to be used for Cubic metric computation for dual carriers, primary carrier and secondary carrier, respectively. This approximation method, basically, reduces the number of considered configurations back to the single carrier case. The circuitry merely needs to compute, or alternatively lookup in a table, the Cubic metrics for each carrier individually, and then combine these in accordance with the equation to approximate the corresponding Cubic metric for the dual- (or more generally multi-) carrier case. FIG. 2 is a graph illustrating coefficients for the affine mapping between Cubic metrics on individual carriers to the Cubic metric for the whole signal.
The above described approximation method works well when the scrambling codes on the two carriers are different, with a typical approximation error that is less than 0.1 dB. However, when the same scrambling code is used on both carriers then the approximation error may be up to about 0.5 dB. The reason is that correlation between the signals on the two carriers has an impact on the Cubic metric. This means that conventional techniques require that a huge number of configurations must be considered to determine the MPR when the same scrambling code is used on both carriers.
It is therefore desired to have methods and apparatuses that overcome the disadvantages determining MPR when several carriers are employed, two or more having the same scrambling code.